1. Field of the Invention
The present invention relates to liquid crystalline compounds and liquid crystal compositions. More particularly, the invention relates to phenyldioxane derivatives, which are liquid crystalline compounds suitable for use as components of liquid crystal compositions, inter alia liquid crystal compositions for TFT (thin film transistor) liquid crystal display elements, as well as to liquid crystal compositions containing the compounds, and to liquid crystal display elements formed through use of the liquid crystal compositions.
2. Description of the Related Art
Liquid crystal display elements utilize the optical anisotropy and the dielectric anisotropy of liquid crystal substances. Based on their display modes, liquid crystal display elements are classified into various types such as TN (twisted nematic) mode, DS (dynamic scattering) mode, guest-host mode, DAP (deformation of vertical aligned phases) mode, and STN (super-twisted nematic) mode. Liquid crystal materials suitable for these types of liquid crystal display elements have different characteristics. Recently, liquid crystal display elements are required to have improved display quality, resulting in increased demand for active-matrix-mode display elements such as a TFT (thin film transistor) liquid crystal display element.
Liquid crystal materials used for these display elements must be stable against moisture, air, heat, light, etc. Further, they must exhibit liquid crystal phases within as broad a temperature range as possible, around room temperature; must have a low viscosity, good compatibility, a large dielectric anisotropy value (.DELTA..epsilon.), and an optimal birefringence value (.DELTA.n); and must exhibit a high voltage holding ratio. Especially, liquid crystal materials used for TFT liquid crystal display elements must have a high voltage holding ratio. However, at the present, no single compound satisfies all of the above-mentioned requirements. Therefore, liquid crystal compositions that are obtained by mixing several kinds of liquid crystalline and non-liquid crystalline compounds are currently used.
Recently, there has arisen a demand to drive TFT liquid crystal display elements at low voltage. This in turn demands liquid crystalline compounds and liquid crystal compositions which have a higher .DELTA..epsilon. as compared to conventional liquid crystal materials (hereinafter, the term "liquid crystalline compounds" is used as a term which encompasses compounds having liquid crystal phases and compounds that have liquid crystal phases even when mixed with other liquid crystals). Therefore, active efforts have been carried out to develop a liquid crystal material having a large .DELTA..epsilon. while maintaining a high voltage holding ratio. Conventionally, as a liquid crystal material exhibiting a high voltage holding ratio, a fluorine-containing compound such as a compound expressed by the following formula (10) is generally known (Japanese Patent Publication (kokoku) No. 1-04496). ##STR2##
The compound of formula (10) has a higher specific resistance than do liquid crystalline compounds having a cyano group, and thus, it is mainly used for a component of liquid crystal compositions for TFT liquid crystal display elements. However, since the extrapolated .DELTA..epsilon. of the compound is as small as 8.7, the compound cannot be used as a liquid crystal material for low voltage drive, such as 2.5 V drive, which is currently demanded.
The above-mentioned extrapolated .DELTA..epsilon. is a value calculated from the .DELTA..epsilon. of a composition containing a base liquid-crystal having a nematic phase and the formula (10) compound dissolved therein, the .DELTA..epsilon. of the base liquid crystal, and the mixture ratio of the compound with respect to the composition. The extrapolated .DELTA..epsilon. substantially reflects the .DELTA..epsilon. of the formula (10) compound (in the following descriptions, .DELTA..epsilon. used in relation to compounds has the same meaning).
As a liquid crystal material having a .DELTA..epsilon. larger than that of the above-mentioned compound of formula (10), a trifluorophenyl derivative expressed by the following formula (11) is known (Japanese Patent Application Laid-Open (kokai) No. 2-233626). ##STR3##
The .DELTA..epsilon. of the formula (11) compound is 11.7, which is greater than the .DELTA..epsilon. of the formula (10) compound. However, this compound also cannot satisfy the market needs for low voltage drive, for the same reason as described above.
Moreover, a trifluoromethylphenyl derivative expressed by the following formula (12) and a trifluoromethoxyphenyl derivative expressed by the following formula (13) are known (Japanese Patent Application Laid-Open No. (kokai) No. 4-506361). ##STR4##
Since the .DELTA..epsilon.s of these compounds are not sufficiently large (for example, the .DELTA..epsilon. of the formula (13) compound is 8.7 (IDY (The Institute of Television Engineers of Japan Techninal Report) 95), these compounds cannot satisfy the market needs for low voltage drive, for the same reason as described above.
There is also known a tricyclic compound expressed by the following formula (14) which corresponds to the above-mentioned formula (13) compound but having two fluorine atoms substituted at lateral positions of the central benzene ring (IDY (The Institute of Television Engineers of Japan Techninal Report) 95). ##STR5##
The .DELTA..epsilon. of the formula (14) compound is 13.0, which is 4.3 greater than the .DELTA..epsilon. of the formula (13) compound. However, this compound also cannot satisfy the market needs for low voltage drive, for the same reason as described above.
Also, a dioxane derivative expressed by the following formula (15) is known as a compound having a large .DELTA..epsilon. (Japanese Patent Application Laid-Open No. (kokai) No. 2-233626). ##STR6##
The .DELTA..epsilon. of the formula (15) compound is as large as about 15. However, the .DELTA..epsilon. of this compound is not sufficiently large to lower the drive voltage of TFT liquid crystal display elements to a required level. Also, since the voltage holding ratio of the compound at 100.degree. C. is as low as 89%, the mixture ratio of the compound cannot be increased for the preparation of a liquid crystal composition for TFT liquid crystal display elements. Accordingly, there have been strong demand for a compound that can solve the above-described problems.
Lastly, a dioxane derivative expressed by the following formula (16) which resembles the compound (15) is also known. However, the .DELTA..epsilon. of this compound has been found to be as small as 9.0 (Japanese Patent Publication (kokoku) No. 64-2114). ##STR7##